Combined cooling, heat and power for the generation of deep-freeze refrigeration

The use of the low-temperature excess heat from a CHP system for the generation of deep-freeze refrigeration in an absorption refrigeration system is sharply constrained by the heating temperature of around 90 °C. On the one hand, single-stage units are only capable of producing cooling in the single-digit minus range, and on the other hand, the thermal ratio of this type of plant is rather low. Moreover, under certain circumstances, the refrigeration of the heating medium will not be sufficient to ensure the cooling of the CHP motor circuit within safe limits, which is why engineering solutions are necessary for the utilisation of the residual thermal volume. The use of multi-stage units may enable the achievement of lower temperatures whilst ensuring sufficient cooling for the CHP plant. Yet, due to the significant amount of additional equipment involved, the use of this type of system is currently restricted to exceptional cases and research environments.

Nevertheless, in comparison with a baseline reference system comprising third-party power supply, compression refrigeration and the use of a natural gas boiler to cover the total steam requirement, it is possible to achieve both economic and ecological benefits through the use of a combination of CHP unit, a waste heat recovery boiler and an absorption refrigeration system.

Because of the high capital investment requirement for the cooling plant and various treatment stages, the economic benefits accrue less from the fact that the absorption refrigeration system obviates the need for cooling from a compression refrigeration unit, and more from the fact that expensive third-party power supply costs are replaced by on-site electricity generation, and that steam can be produced in a waste heat recovery boiler in an extremely cost-effective manner.

The ecological benefits of this type of system are the result of effective fuel exploitation and the fact that the primary energy factor for natural gas is significantly lower than that of electrical power. Moreover, because third-party power consumption is reduced, the company's expenditure on primary energy is also reduced.

In the case of a CHP system with an electricity generation capacity of 2 MW, which also contributes to the provision of saturated steam at 10 bar(g) by way of a waste heat recovery boiler, and provides refrigeration at around -7 °C by way of an absorption refrigeration system, a primary energy cost saving in excess of 15 % is more than feasible! Should the saturated steam be required at a lower pressure or the cooling at a higher temperature, then the system efficiency level of the CCHP plant will increase, which will, in turn, result in a greater primary energy cost saving.

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